Wind turbines can be generally divided into two types: ‘horizontal axis’ wind turbines (HAWT) haying an axis of rotation arranged to be aligned with prevailing wind direction; and cross-flow or ‘vertical axis’ wind turbines (VAWT) having an axis of rotation arranged to be generally perpendicular to the prevailing wind direction.
Within the VAWT category, turbines generally fall into one of two further categories: drag-type and lift-type.
Drag-type turbines, of which the most well known is the Savonius wind turbine, operate by having blades which each have a greater drag when moving with the wind than when moving into the wind. This difference in drag, induces a torque about an axis, causing rotation of the blades about the axis. Lift-type turbines, such as Darrieus turbines, use airflow about the blades to generate a lift force on the blades, which is translated into rotational movement.
Drag-type blades are necessarily limited to travelling slower than the prevailing winds. Lift-type blades, however, can reach a tip speed ratio (velocity of blade tip: prevailing wind velocity) well in excess of 1.0.
Lift-type VAWT are generally significantly more efficient in producing power than drag-type VAWT. Nonetheless, various problems have been experienced in producing efficient VAWTs for commercial applications.
One problem is in turbine start-up, and operation in light winds. Lift-type turbines rely on the lift provided by the rotating blade—thus rotation of the turbine requires a certain speed to be self sustaining. This may require the provision of a motor to start the turbine. Similarly, if the wind speed is not sufficient to maintain the minimum turbine rotation speed, the turbine will stop.
Another problem experienced is in the spacing of adjacent turbines. Wind turbines generally operate best in ‘clear’ air, without nearby obstacles which could act to slow or otherwise disturb wind flow. For this reason, it is generally necessary to space wind turbines so that they do not interfere with each other's air flow. One way around this spacing problem is to incorporate a cutaway or ‘cupped’ feature into the inner face of each turbine blade. The idea behind this concept is that the turbine will use the difference in drag between the outer and inner surfaces of the blade to generate torque at low speeds, with this torque being sufficient to accelerate the rotor to a speed where sufficient rotational torque is generated so as to drive an electrical generator.